quinta-feira, janeiro 23, 2014

Seven things about evolution

John Hawks

What is evolution?

In its original sense, evolution meant "unrolling", as if a papyrus scroll were being unrolled to reveal its contents. We may talk about the "evolution" of many things, from an individual's lifetime to the evolution of the universe. In the most general sense, evolution means "change".

Biologists are very specific about the kinds of processes that qualify as "evolution" in the biological sense. Biological evolution is genetic change in a population over time. Populations and individuals change in many ways, but only some changes are evolution.

Here's a list of seven things about evolution. It's not comprehensive but it hits on several important issues that help to understand how evolutionary biologists think about the process of evolutionary change.

Evolution is change in a population. Individuals change during their lifetimes, even day to day. Those changes are not biological evolution, although they may be products of evolution in past populations. Likewise, a forest may change over time, as some kinds of trees proliferate and others disappear. Those changes in community structure are not themselves biological evolution, although they may influence the evolution of the populations of trees composing the forest.

Evolution is genetic change. Many kinds of phenotypic changes don't involve evolution. For example, many human populations have markedly increased in lifespan during the last 100 years, mostly as a result of improvements in nutrition and reductions in disease. Those changes are important and highly visible, but they are not biological evolution. Physical characteristics and behaviors can only evolve if they have some genetic contribution to their variation in the population -- that is, if they are heritable.

Many kinds of genetic changes are important to evolution. Mutations happen when a DNA sequence is not replicated perfectly. A sequence may undergo a mutation to a single nucleotide, small sequences of nucleotides can be inserted or deleted, large parts of chromosomes can be duplicated or transposed into other chromosomes. Some plant populations have undergone duplications or triplications of their entire genomes. These patterns of genetic change can have a wide range of effects on the physical form and behavior of organisms, or may have no effects at all. But all of them follow the same mathematical principles as they change in frequency within populations.

Evolution can be non-random. Populations of organisms cannot grow in numbers indefinitely, so that individuals that successfully reproduce will have their genes increase in proportion over time. Among the genes carried by such successful individuals may be some that actually cause them to survive or reproduce, because they fit the environment better. The survival and proliferation of such genes is not a matter of chance; it is a result of their value in the environment. This process is called natural selection, and it is the reason why populations come to have forms and behaviors that are well-suited to their environments.

Evolution can be random, too. Many genetic changes are invisible and make no difference to the organisms. Many changes that do make a noticeable difference to the organisms' form or behavior nevertheless still do not change the chance of reproducing. Even individuals with the best genes still have a strong random component to their reproduction, and in sexual organisms genes assort randomly into sperm and egg cells. As a result, even when an individual has a beneficial gene that increases the chance of reproducing, that valuable gene still is very likely to disappear quickly after it first appears in the population. Genetic drift is strongest when populations are small or genes rare, but it is there all the time. Random chance has a continual role in evolutionary change.

Populations evolve all the time. No population can stay static for long. Reproduction is not uniform, and no organism replicates DNA perfectly. The genome of the simplest bacterium has thousands of nucleotides, ours has billions. Keeping these sequences constant, generation after generation, is a task no population has ever managed to do. Genetic variation is constantly introduced into populations by mutation and immigration, rare genetic variations are constantly disappearing when individuals who carry them don't pass them on, and occasionally rare genes become common -- whether by natural selection or genetic drift. If a population's physical form remains the same for a long time, we have a good reason to suspect that natural selection is working to oppose random changes.

Evolutionary theory has changed a lot since Darwin's day. Charles Darwin recognized several key insights about biological evolution, including the process of natural selection, the tree-like pattern of relationships among species, and the potential for significant changes when processes act through small, incremental steps across geological timescales. But we know a lot more now than Darwin knew. We understand the molecular basis of genetic changes, and many of the ways that the features of organisms can be affected by genetic and environmental change. We have learned much about the limits of evolution, the alternative patterns of change caused by environments, and the importance of randomness. We now know much about the changing pace of evolution, seeing it as a dynamic process that can happen in fits and starts.

Evolution is the most powerful idea in biology, organizing our knowledge about the history and diversity of life. We understand our own origins using the same tools that we use for organisms across the tree of life, from the simplest bacteria to the largest whales.